Recently, noises and vibration problems have become an object of public concern as an environmental pollution with development of means of transportation and increase in residential areas which are located near factories and the like. Further, in a workshop, there is a tendency to limit noises and vibration to improve working atmosphere. To cope with these tendencies, it is requested to impart vibration damping property to a metallic material which is a source of noises and vibration, that is, to impart a function to a noise generating member itself so that the member can absorb its own vibrational energy and convert it into heat energy to attenuate frequency of vibration or vibrational amplitude, whereby noise is decreased. Further, it is requested to improve such a function.
Based on these requests, as one of vibration damping materials having desired property, there has been proposed a vibration damping material having a composite laminate structure wherein a middle layer having viscoelastisity is sandwiched by metal layers. This type of a composite vibration damping material has been studied and employed as oil pans of automobiles, engine covers, chutes of hoppers, stopper of conveying apparatus, domestic electric equipments, vibration reducing members of other metal processing machines, structural members of precision machines in which prevention of vibration is desirable and the like.
In general, vibration damping property of such a composite vibration damping material depends upon the property of a viscoelastisity layer which constitutes the middle layer thereof. When vibration damping property is expressed as a loss factor (which is a measure of conversion of an external vibrational energy into a heat energy by internal friction, and is corresponding to a value relating to mechanical hysteresis loss due to vibration), the property shows a peak at a certain temperature. It has been known that it is most effective to use a vibration damping material at about this temperature showing the peak property.
Hitherto, as a viscoelastic composition which constitutes the middle layer of such a composite vibration damping material, there have been known a simple polyester (Japanese Patent Kokai No. 50-143880) or a polyester to which a plasticizer is added (Japanese Patent Kokai No. 51-93770): a simple polyurethane foam (Japanese Patent Kokai No. 51-91981): a simple polyamide (Japanese Patent Kokai No. 56-159160); a simple ethylene-polyvinyl acetate copolymer (Japanese Patent Kokai No. 57-34949): a composition of a polyvinyl butyral or a polyvinyl butyral and a polyvinyl acetate to which a plasticizer and a tackifier are added (Japanese Patent Kokoku No. 55-27975): a copolymer of a isocyanate prepolymer and a vinyl monomer (Japanese Patent Kokoku No. 52-26554): copolymers disclosed in Japanese Patent Kokoku Nos. 39-12451 and 45-34703: and the like.
Although, first of all, it is required that a composite vibration damping material should have a high value of the above loss factor as well as a high adhesive strength between a viscoelastic middle layer and a metal layer, the composite vibration damping material made of the above known viscoelastic composition has problems in any of these properties and is unsatisfactory. Further, although, in addition to the above requisite properties, it is necessary that a composite vibration damping material should stand processing such as press, bending and the like. A composite vibration damping material made of the above conventional viscoelastic composition is liable to produce wrinkle, crack and the like, and is also unsatisfactory.